(1) Field of the Invention
This invention relates to deodorization and/or wastewater treatment utilizing a photocatalytic reaction. More specifically it relates to a composite structure having a photocatalytic function, which is capable of decomposing smelly or harmful substances contained in air or other gas and/or wastewater effectively, and to the utilization thereof.
(2) Description of the Related Art
In recent years, researches and developments have been made actively for utilizing a photocatalytic function possessed by titanium oxide and other metal oxides. That is, these metal oxides such as titanium oxide and the like have excellent functions for preventing contamination by removing harmful substances, purifying the air by decomposing ammonia, a sulfur compound and the like contained in the air, sterilizing a kind of bacteria and so on. Therefore, applications of these metal oxides are expected in various fields.
Finely divided titanium oxide particles having a photocatalytic function (photocatalytic property), (hereinafter, photocatalytic titanium oxide particles are simply referred to as xe2x80x9ctitanium oxidexe2x80x9d when appropriate), are used in a variety of forms such as bulk particles, sols, thin films and the like depending on purposes. However, in the field of deodorization and wastewater treatment, titanium oxide particles are usually used not alone, but used in combination with a substrate such as, for example, glass, ceramics or the like in such a form supported on the substrate in many cases. Many methods of supporting titanium oxide particles on a substrate have been proposed. In order to enhance adherence of titanium oxide particles to a substrate and to prevent detachment of titanium oxide particles from the substrate, the following methods are generally employed: (i) after a titanium oxide coating film being formed on a substrate, heat-treating the film to sinter on the substrate at high temperatures; (ii) impregnating a dispersion of titanium oxide particles into pores of a porous substrate; and (iii) forming a titanium oxide coating film on a substrate through a binder.
However, in the case of method (i) of heat-treating a titanium oxide coating film to sinter on a substrate at a high temperature, the titanium oxide particles grow largely to lower their specific surface area during heat treatment. Consequently, their photocatalytic function becomes decreased. In the case of method (ii) of impregnating a dispersion of titanium oxide particles into pores of a porous substrate, it is difficult to prevent sufficiently the detachment of the titanium oxide particles. The method (iii) of forming a titanium oxide coating film on a substrate through a binder is generally employed. However, this method is still unsatisfactory.
Considering the aforementioned background art, an object of the present invention is to provide a composite structure for deodorization and/or wastewater treatment having finely divided titanium oxide particles having a photocatalytic function, supported on a substrate, which exhibits great photocatalytic activity, thereby having improved deodorization and/or wastewater-treatment properties, in general, the finely divided titanium oxide particles of which are hardly detached from the substrate to exhibit an enhanced photocatalytic property over a long period of time.
The present inventors made extensive researches into a composite structure having a photocatalytic function, which is capable of decomposing effectively smelly or harmful substances contained in the air or wastewater. As a result, it was found that the above-objects can be satisfied by a composite structure containing finely divided titanium oxide particles adhered to the surface of a foamed or porous substrate having small apparent specific gravity. The present invention has been completed on the basis of the above-finding.
Thus, in one aspect of the present invention, there is provided a composite structure, which has a photocatalytic function and can be used for deodorization and wastewater treatment, comprising a foamed or porous substrate having apparent specific gravity of 0.9 to 0.01 and finely divided titanium oxide particles which are adhered onto the surface of the foamed or porous substrate.
In another aspect of the present invention, there is provided a method for deodorizing air or other gas having offensive odor or treating wastewater, comprising the step of allowing air or other gas having offensive odor or wastewater to be in contact with a composite structure having a photocatalytic function, which comprises a foamed or porous substrate having apparent specific gravity of 0.9 to 0.01 and finely divided titanium oxide particles adhered onto the surface of the foamed or porous substrate, whereby smelly or harmful substances contained in the air or other gas or wastewater are decomposed.